The vulnerability of some women to develop major depressive disorder (MDD) is associated with hormonal fluctuations. It is likely that effects of gonadal hormones contribute to the development of MDD (as well as postpartum depression) and this may be part of the reason why MDD is more common in females than in males. In spite of this, preclinical research examining the effects of antidepressants, particularly selective serotonin reuptake inhibitors (SSRIs)- the drug class of choice for the treatment of MDD- on either behavioral or neurochemical parameters, have mainly used male subjects. In our experiments, effects of SSRIs in female rats were examined. It was found, using the technique of in vivo chronoamperometry, that acute administration of ovarian hormones (estradiol benzoate (EB) or progesterone (P)), either given systemically or locally into the CA3 region of the hippocampus, interferes with the ability of SSRIs to inhibit the function of what is widely considered their initial cellular target in brain- the serotonin transporter (SERT). In addition, EB but not P blocked the function of the SERT. Further, there appears to be some difference in the mechanism(s) mediating these two effects of estradiol. Further, these two effects of EB would be expected to exert counteracting effects on the ability of estradiol, or estradiol plus progesterone, to improve the efficacy of SSRIs when given in combination with such treatment. Evidence was obtained that these effects of estradiol are mediated via activation of membrane as well as nuclear estrogen receptors (ER). By contrast, the effect of progesterone seems to be mediated primarily by nuclear receptors. These previous results provide the rationale for the studies in the current proposal, whose principal goal is to extend these studies using neurochemical or behavioral measures to see if further evidence can be obtained that treatment with female sex hormones either influences SERT function and/or interferes with the ability of SSRIs to inhibit the SERT. Experiments will be carried out in ovariectomized female rats. The new techniques to be used are in vivo microdialysis and the forced swimming test (FST). Our hypotheses are that acute treatment with either estradiol or progesterone will block the ability of locally applied fluvoxamine, an SSRI, either to elevate extracellular levels of serotonin in the hippocampus or its ability to decrease immobility or increase swimming behavior in the FST. In addition, to examine possible mechanisms underlying the hormonal effects: (1) biotinylation studies on synaptosomal membranes will be carried out to examine the plasma membrane distribution of the SERT after hormone treatment; (2) using chronoamperometry, two approaches will be used, namely the use of specific estrogen receptor (ER1 or ER2) knockout mice and the effects of selective estrogen receptor agonists in rats, to study the role of these estrogen receptor subtypes in mediating the effects of estrogen; and again using chronoamperometry (3) the possible involvement of brain-derived neurotrophic factor (BDNF) as an intermediary in the effect of estradiol on the action of an SSRI will be examined by using an antagonist of the receptor for BDNF. Finally, we will investigate if longer-term, more clinically appropriate treatment with E2 alone or E2+P alters SERT function and/or effects produced by an SSRI given chronically.